Process of management of a thermochemical reaction or of a solid-gas adsorption

ABSTRACT

The invention concerns a method for controlling a thermochemical reaction or a solid-gas adsorption being carried out in a reactor ( 10 ) containing an active agent capable of reversibly reacting with a gas, the reactor ( 10 ) being connected to an evaporator/condenser assembly ( 14 ) for the gas by a connection ( 12 ) without control valve, the reactor ( 10 ) and the evaporator/condenser assembly ( 14 ) each provided with means for selectively exchanging calories with their surroundings. The control method consists in: thermally insulating the reactor ( 10 ) and the evaporator/condenser assembly ( 14 ) from the surroundings; thermally communicating the reactor ( 10 ) with its surroundings so that the active agent reacts with the gas, thereby providing cold to the evaporator ( 14 ); thermally communicating the evaporator ( 14 ) with its surroundings so as to cool it selectively; and selectively insulating the reactor ( 10 ) or the evaporator/condenser assembly ( 14 ) from their surroundings so as to stop the reaction at one point of the reversible cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process of management of athermochemical reaction or of a solid-gas adsorption permitting theproduction of cold and/or of heat.

2. Description of the Related Art

A thermochemical reaction, or an adsorption, is based on a reversiblereaction between a solid and a gas, and can be schematized by theequation:

In the reactor, the reaction is exothermic in the direction 1, whichmeans that it produces heat, and it is endothermic in the direction 2.In the direction 1, it likewise produces cold in the associatedevaporator by the evaporation of the gas (G).

Such a system permits the storage of energy in a chemical or physicalform, and has varied fields of application.

In a conventional manner, this type of thermochemical or adsorptionreaction is located in a system comprising an enclosure, termed a“reactor”, containing a salt or an adsorbent and, preferably, anexpanded binder which is a good conductor of heat. The reactor isarranged so as to be capable of being selectively placed incommunication with a second enclosure forming an evaporator/condenserassembly for the gas intended to react with the salt. This communicationis effected by a duct provided with a control valve. During aconventional cycle of reaction or of adsorption, the valve is open,permitting the gas present in liquid form in the evaporator to beevaporated and to pass through the duct in order to react with the saltor the adsorbent present in the reactor, resulting in the cooling of theevaporator. At the end of the evaporation phase, the salt or theadsorbent in the reactor is heated, for example by means of anelectrical resistance, thus causing the discharge of the gas toward thecondenser. The control valve permits stopping the reaction cycle at anymoment.

In order to permit a continuous production of cold and/or of heat, twoanalogous sub-assemblies can be placed side by side, one producing coldand/or heat while the other is in a regeneration phase.

An example of a thermochemical system of this type, having twosub-assemblies, is described in the document EP-A-0 382 586. In thissystem, the evaporator/condenser assembly of each sub-assembly comprisesa reactor able to absorb or desorb the gas. The ducts connecting thereactors are each provided with a control valve for the passage of gas.The control valves form the sole means of setting in operation or ofstopping the thermochemical reaction or the adsorption/desorption.

The use of control valves in the ducts connecting the reactors has twodisadvantages.

Each valve requires an associated actuation for opening or closing it,and this increases the cost and/or the complexity of the system. But inaddition, the presence of valves increases the risk of leaks of gas fromthe system. This risk of leaks of gas, for example of ammonia, reducesthe number of practical applications of this type of thermochemicalsystem.

SUMMARY OF THE INVENTION

The present invention thus has as its object a process of management ofa thermochemical reaction or of an adsorption/desorption which does notrequire the presence of any control valve in the thermochemical system.

In the case in which a thermochemical system is intended to producecold, it comprises for this purpose a single reactor connected to anevaporator/condenser assembly. If, when this system is in a phase ofproduction of cold, the reaction is stopped by closing a control valve,the temperature of the evaporator will tend to increase due to theeffect of the ambient air while the reaction is stopped. Then, when thereaction is restarted by reopening the valve, there is a time duringwhich the thermochemical reaction only serves to reduce the temperatureof the evaporator down to its level before the stop. The energy of thethermochemical reaction is thus lost during this phase.

The present invention has as its second object a process of managementof a thermochemical reaction which ensures that, during each stop, thetemperature of the evaporator substantially does not vary, or variessolely in a favorable manner.

In order to attain these objects, the present invention proposes aprocess of management of a thermochemical reaction or of a solid-gasadsorption located in a reactor containing a reactive agent able toreact in a reversible manner with a gas, the reactor being connected, bya connection devoid of a control valve, to an evaporator/condenserassembly for the gas, the reactor and the evaporator/condenser assemblyeach being provided with means permitting selective exchange of calorieswith the environment, characterized in that the process of managementcomprises the steps consisting of:

thermally isolating the reactor and the evaporator/condenser assemblyfrom their environment;

placing the reactor in thermal communication with its environment inorder for the active agent to react with the gas, with the production ofcold at the evaporator;

placing the evaporator in thermal communication with its environment inorder to selectively cool the latter; and selectively isolating thereactor or the evaporator/condenser assembly from their environment inorder to stop the reaction at a point of the reversible cycle.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

The advantages, as well as the operation, of the present invention willbecome apparent more clearly on reading the following description, withreference to the accompanying drawings in which:

FIG. 1 is a schematic view of a thermochemical system permitting theembodiment of the process according to the invention; and

FIGS. 2-4 each show a Clapeyron diagram illustrating a phase of theprocess according to the invention, in the case of univariant reactionsat equilibrium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a thermochemical system comprises a reactor 10intended to contain an active agent capable of reacting in a reversiblemanner with a gas. Preferably, the active agent comprises a salt, thegas being ammonia. In a preferred embodiment, the salt is dispersed in abinder comprising expanded graphite, which may be recompressed. Thereactor 10 is connected via a duct 12 to an evaporator/condenserassembly 14. Furthermore, the reactor is provided with a reheating means16 intended to permit the thermochemical reaction between the salt andthe gas to proceed in the direction of a regeneration. Preferably, thereheating means comprises an electrical resistance. A sleeve 18 ofthermally insulating material is disposed around the reactor 10. Theends of the sleeve 18 are open, thus permitting air, blown by a fan 20,to pass around the reactor 10 in the direction of the arrows 22. Asleeve 24, likewise of an insulating material, is disposed around theevaporator/condenser assembly and has open ends, analogously permittingair, blown by a fan 26, to pass around the assembly in the direction ofthe arrows 28.

According to a first aspect of the invention, the duct 12 is notprovided with a control valve, the passage between the reactor 10 andthe evaporator/condenser being permanently open.

The process of management of the thermochemical reaction, located in thesystem of FIG. 1, will now be described.

In the Clapeyron diagram of FIG. 2, there are shown the equilibriumstraight lines of the univariant liquid-gas transformations of ammonia,labeled NH₃, and univariant solid-gas transformations of a salt, labeled“salt”, which reacts with ammonia. If the two elements 10 and 14 of FIG.1 were separated by a valve placed in the duct 12, the equilibriumpressure and temperature of these two elements at ambient temperature T₀would be those defined at A and B respectively. Considering the case inwhich the duct 12 is not provided with a control valve and in which thesystem has just produced cold at the temperature T_(B) (see FIG. 3), theevaporator is situated at the point B and the reactor is located at thepoint D, with a separation with respect to the equilibrium straight lineof the salt. Toward the end of the thermochemical reaction, when all theammonia has reacted with the salt, the pressure in the system is set bythe reactor. Thus the pressure of the system will fall toward the pointD′, the evaporator tending toward the point B′.

When it is desired to regenerate the system, the desired temperatureT_(DEC) at the reactor is set, bringing about a rise of pressure topermit the decomposition of the salt, ammonia being discharged towardthe condenser which is then at the point E, the reactor being situatedat the point F.

At the end of the regeneration phase, equilibrium is reached, bringingabout a rise of the pressure of the system which comes to be stabilizedat the points G for the condenser and H for the reactor, as long as thereactor is at the temperature T_(DEC) (see FIG. 4).

When it is desired to produce cold at the evaporator, it is sufficientto stop maintaining the reactor temperature at T_(DEC). When thistemperature is no longer maintained, the system begins to cool. Theevaporator and the reactor respectively follow the paths GI and HJ, thepoint I corresponding to the equilibrium of the evaporator at thetemperature T₀.

As the reactor continues to cool, the temperature T_(J) of the point Jbeing greater than T₀, there follows a fall of the pressure of thesystem, leading the reservoir to pass below T₀, and thus to producecold. The evaporator will produce cold at the point B, with a deviationwith respect to the equilibrium straight line, and the reactor tendstoward the point D. The positions B and D depend on the nature of theheat exchanges associated with the evaporator and with the reactor, andthus on the exchanged heat flows. At the end of this phase of productionof cold, in which the salt contained within the reactor is deprived ofammonia, the system is situated at its starting point, shown in FIG. 3.

In order for the thermochemical system of FIG. 1 to be able to followthe reaction cycle described hereinabove, without having a control valvein the duct 12 connecting the reactor 10 to the evaporator/condenserassembly, a management process is carried out according to the presentinvention.

As the reactor 10 and the evaporator/condenser assembly 14 aresubstantially isolated from the ambient air by their respective sleeves18, 24, the exchange of calories can be managed by selectively passing acurrent of air within the sleeves by means of the fans 20 and 26. If theexchange of calories is prevented, the thermochemical reaction willeither stop, or proceed very slowly.

For example, if it is desired to vary the production of cold, afterpassing from the points GH to the points BD in the operating cycledescribed hereinabove, it is sufficient to control the fan 26 in orderto control the exchange of calories between the evaporator and the air.If the ventilation is interrupted, it will lead to the paths of theevaporator from the point B toward the equilibrium straight line at B′and of the reactor from the point D toward the equilibrium at thetemperature T₀, that is, the point D′. The points B′ and D′ correspondto positionings which are slightly out of equilibrium, these deviationsbringing about a very slight evaporation and/or synthesis reactionwhereby the production of cold and the production of heat are onlypossible by losses through the insulation of the elements 10 and 14.When it is again desired to produce cold, starting the fan again bringsthe system back to the points B and D.

In an analogous manner, when the system is in the regeneration phase,during which phase ammonia is condensed in the evaporator/condenserassembly, the heat of condensation can be selectively withdrawn bystarting the fan 28. As in the preceding example, if the ventilation isinterrupted, the condenser, no longer able to release its heat ofcondensation, stops the thermochemical reaction.

In the two preceding examples, when the reaction is stopped bypreventing the system exchanging calories with the ambient air, the factthat the duct remains open between the reactor and theevaporator/condenser assembly has the result that the thermochemicalreaction proceeds, albeit very slowly. The reaction thus tends tocompensate solely for the calories lost by the system through theinsulation of the sleeves. The temperatures of the reactor and of theevaporator/condenser assembly thus tend to remain constant.

The process of management according to the invention permits thereaction to be stopped at a predetermined point of the cycle during agiven time, with the reaction consuming only the small quantity ofammonia necessary to compensate the thermal losses. Also, the system canbe maintained at a given point of its cycle, the cycle being able torestart at any moment simply by setting one or both of the fans inoperation. This waiting period consumes only a little ammonia.

Thus, by selectively heating the reactor and/or by controlling theexchange of calories between the reactor or the evaporator/condenserassembly and the ambient air, the thermochemical reaction can be managedin a system in which the duct for the gas remains permanently open.

Instead of the insulating sleeve, a sheath of insulating material can beused, disposed in contact with the exterior surface of the reactor.Instead of providing fans, it would be sufficient to withdraw the sheathfrom the reactor, in order to expose it to the ambient air. The rate ofcooling of the reactor would be a function of its surface area exposedto the ambient air. In an alternative manner, the reactor can bedisposed in a closed, insulating enclosure such as a Dewar vessel. Inthis case, the reaction takes place very slowly, solely compensating forthe calories lost through the insulating wall.

When the reaction takes place intermittently, the stopped periods can bemade use of to regenerate the reaction, even if the reaction has notbeen completely terminated. In this manner, a continuous system isapproached in which there is no long regeneration time.

The insulating sleeves 18 and 24 can likewise be arranged to make theminto chimneys with draft, which do not require fans but operate solelyby convection. In this case, the sleeves are advantageously providedwith valves or shutters in order to be able to close them selectively,thus stopping the exchange of calories with the ambient air during theregeneration phase. The natural convection in the chimney around thereactor can be made use of to produce a small quantity of cold at theevaporator during a long period.

To complete the description hereinabove, an embodiment example of theprocess according to the invention is given hereinafter, but nolimitation is thereby implied.

EXAMPLE

In order to cool an insulated enclosure having a volume of 80 liters,the ambient temperature being 26° C., a reactor is disposed around theenclosure, the said reactor having a volume of 4 liters and containing460 g of MnCl₂ mixed with 260 g of expanded graphite. The evaporatorcontains 250 g of NH₃. The air within the enclosure is recirculated overthe evaporator by fans. A bulb placed within the enclosure controls thestarting and stopping of the fans associated with the evaporator and thereactor, with respect to a reference temperature. When the temperatureof the enclosure exceeds the reference temperature, the fans aresupplied with power, the reactor starts to react, and the enclosure iscooled. When the temperature of the enclosure is again below thereference temperature, the fans are stopped. During the operating cycle,the fans are started every 12 minutes for a period of 5.6 minutes.

The evaporator/condenser assembly can be replaced by a second reactorcontaining another salt or another mixture of salts.

Management analogous to that described hereinabove can likewise beapplied to a thermochemical system which is intended to produce heat. Inthis case, the heat is used which is produced at the temperature T_(D),greater than the temperature T₀, the evaporator drawing the necessarythermal energy from the ambient air.

The process of management according to the invention is thus applied tothermochemical reactions, and to solid=gas adsorptions, but likewise toliquid-gas absorptions.

What is claimed is:
 1. Process of management of a thermochemicalreaction or of a solid-gas adsorption located in a reactor containing areactive agent able to react in a reversible manner with a gas, thereactor being connected, by a connection devoid of a control valve, toan evaporator/condenser assembly for the gas, the reactor and theevaporator/condenser each being provided with means permitting selectiveexchange of calories with the environment, characterized in that theprocess of management comprises the steps consisting of: thermallyisolating the reactor and the evaporator/condenser assembly from theirenvironment; placing the reactor in thermal communication with itsenvironment in order for the active agent to react with the gas, withthe production of cold at the evaporator; placing the evaporator inthermal communication with its environment in order to selectively coolthe latter; the process being characterized in that it comprises thestep of isolating, selectively, solely the evaporator/condenser assemblyfrom its environment in order to stop the reaction at a point of thereversible cycle.
 2. Process according to claim 1, characterized in thatthe steps of placing the evaporator or the reactor in thermalcommunication with its environment takes place by convection.
 3. Processaccording to claim 2, characterized in that the steps of placing theevaporator or the reactor in thermal communication with its environmenttake place by a circulation of air.
 4. Process according to claim 3,characterized in that a thermochemical reaction is set in operationbetween a salt and a gas, particularly ammonia, the salt being dispersedin a binder comprising expanded graphite.
 5. Process according to claim2, characterized in that a thermochemical reaction is set in operationbetween a salt and a gas, particularly ammonia, the salt being dispersedin a binder comprising expanded graphite.
 6. Process according to claim1, characterized in that the steps of placing the evaporator or thereactor in thermal communication with its environment take place by acirculation of air.
 7. Process according to claim 6, characterized inthat the air is circulated by means of fans.
 8. Process according toclaim 7, characterized in that a thermochemical reaction is set inoperation between a salt and a gas, particularly ammonia, the salt beingdispersed in a binder comprising expanded graphite.
 9. Process accordingto claim 6, characterized in that a thermochemical reaction is set inoperation between a salt and a gas, particularly ammonia, the salt beingdispersed in a binder comprising expanded graphite.
 10. Processaccording to claim 1, characterized in that a thermochemical reaction isset in operation between a salt and a gas, particularly ammonia, thesalt being dispersed in a binder comprising expanded graphite.